The present invention generally relates to articles of manufacture and methods of fabrication for mechanisms to relieve hoop stress in rotating bodies and, more specifically, to a J-slot modification to a rotating disk having integral cast blades such as those contained in a gas turbine engine.
Hoop stress is defined as a load measured in the direction of the circumference of a rotating body, the load being created by thermal gradients and centrifugal forces acting in a radial direction outwardly from the axis of rotation of the body. Such stress is particularly acute in the art of gas turbine engine design where the turbine disks may have integrally cast blades. Such turbine disks have been observed to develop fractures along the circumference of the disk during use.
A number of methods were devised to prevent such fractures. Initially a series of circumferential slots were fabricated into the outer edge of the disk and extending inwardly, the slots being produced using an electric discharge wire machine (EDM). The slots were observed to develop fractures at the inner end nearest the axis of rotation during use, so that a relief hole was drilled at the inner end of the slot to prevent further fracturing. The relief hole was observed to promote increased hot gas ingestion through the disk, so that a rivet or pin had to be inserted through the hole to block such gas ingestion.
This particular prior art hoop stress relief mechanism is shown in FIGS. 1 and 2. According to FIG. 1, a prior art hoop stress relief mechanism is shown as fabricated into a section of rotary body 100 with integral blades 110. A slot 120 may be cut into the rotary body 100 radially from an outer rim 130 to intersect with a hole 140. The slot 120 and hole 140 extend through the rotary body 100 so that the face 150 and the opposing face (not shown) of the rotary body 100 may be connected. A rivet 160 shown in phantom line may be inserted into the hole 140 and secured, so that hot gasses impacting face 150 may be prevented from flowing through the hole 140 to the opposing face of the rotary body 100. Fabrication of the prior art hoop stress relief mechanism as shown may comprise the steps of drilling hole 140 through face 150 of the rotary body 100, using an EDM to fabricate a continuous slot 120 from the outer rim 130 to the hole 140, and deburring and reaming the hole 140 so that any gouges in the walls of hole 140 may be prevented from serving as sites for fractures in the rotary body 100. Referring to FIG. 2, a plurality of prior art slots 120 and holes 140 are fabricated between blades 110 around the circumference of the rotary body 100 so that hoop stress may be reduced and evenly distributed about the entire circumference of the outer rim 130.
The method for fabricating this hoop stress relief mechanism involves a number of manufacturing steps. Referring to FIG. 1, the hole 140 must be first drilled through the rotary body 100 and then reamed to remove any objectionable grooves or defects in the hole walls that formed in the drilling process. Next, the slot 120 must be machined from the outer rim 130 of the rotary body 100 to intersect the hole 140. A rivet 160 must then be installed in the hole 140 to inhibit the flow of hot gasses through the hole 140 thus formed. These steps are used to fabricate the hole-and-slot configuration about the outer rim 130 of the rotary body 100, and then the rotary body 100 is spun and balanced. The rivets 160 must then be inspected after the spinning operation to ensure that they are still properly seated and not deformed by the centrifugal force generated by the spin operation.
However, there are a number of problems associated with this mechanism: First, the method of fabricating the hoop stress relief mechanism involves a detailed sequence of operations that must be precisely executed. This sequence consists of drilling a hole of exact proportions through the disk, reaming the hole to eliminate ridges and grooves within the walls of its bore, cutting an EDM slot from the rim of the disk to the hole, inserting a rivet through the hole to prevent hot gas ingestion from an adjacent space, and inspecting the rivet for correct installation and placement. This sequence is labor intensive, time consuming, and exacting, and thus expensive.
Second, the rivet inserted into the drilled hole is frequently dislodged by vibration, thermal shock, or mechanical means during use. The rivet thus released can cause downstream damage within the turbine. Also, hot gases may subsequently leak through the turbine disk and reduce engine efficiency.
A third problem is that rivets have varying tolerances, so that when installed, they present a balancing problem. As the turbine rotates more rapidly, rivets that are mismatched as to size, weight, or placement along the circumference of the disk start producing unacceptable vibration. Too much vibration can cause the entire turbine to fail.
A fourth problem is that there are variations between different tools used to fabricate the holes and slots, which must be accounted for. For example, in a test, 24 holes were drilled with a 0.120″ drill, reamed with a 0.128″ reamer, and finally finished by four 0.1315″ reamers (6 holes each) to determine if tool variation was significant. An analysis of variance of the surface finish as a function of the block (final reamer) yielded a p-value <0.05, that is, the confidence is greater than 95% (p-value is a statement of probability where confidence=1−p-value). This test showed that the reamer/tool is significant and influences the surface finish. Mean surface finish for each tool ranged from 9.8Ra to 25.9Ra. Therefore, hole-drilling quality is limited by tool variation and is a problem in production fabrication. Current hole drilling processes impart detrimental flaws to the inner diameter surface of the rivet hole; these flaws can serve as sites at which fractures are initiated.
A number of similar methods have been found in the prior art to relieve hoop stress in various engine parts. U.S. Pat. No. 3,781,125 teaches the use of a keyhole shaped slotted portion in the outer shroud structure of a nozzle vane structure for a gas turbine engine. The keyhole shaped slot reduces stress due to larger temperature gradients. A threaded sealing member, instead of a rivet, is inserted into the keyhole to restrict gas flow. However, this application is made for a non-rotating shroud, and not for a turbine disk, and therefore does not experience the same problems as would be experienced by a rapidly rotating turbine disk.
U.S. Pat. No. 4,536,932 teaches a method of forming a turbine disk having integral blades from a plate shaped forging preform A plurality of slots is formed between the integral blades, and the slots are then closed by the forging process. A rod or wire may be inserted at the base of each slot to increase the radius at the end of the slot. However, this process is amenable only to forging processes and does not address machining issues regarding the slot bases.
U.S. Pat. No. 5,071,313 teaches the use of T-shaped relief slots of a shroud body of a gas turbine engine. The relief slots are made in the outer portion of a non-rotating shroud for relief from thermal stress and not for centrifugal stress, where balancing and uniformity of the slots is a concern. The teaching is made for a non-rotating shroud, and not for a turbine disk, and therefore does not experience the same problems as would be experienced by a rapidly rotating turbine disk. The teaching does not discuss any considerations in the fabrication of the slots.
As can be seen, there is a need for a mechanism for relieving hoop stress in a rapidly rotating turbine disk structure, where the mechanism is simple to fabricate, does not allow excessive passage of hot gasses through the turbine disk, does not employ rivets which may become dislodged through use, and does not depend upon uniformity of the machining tools used to fabricate the mechanism.